Observation data mixup due to observation token re-use

[GiedriusM]

Restserver may send invalid resource value when observations are deleted and added.

Bugflow:
C - LwM2m client; S - LwM2M server (wakaama); R - REST client;

CS: Registration
RS: Subscribe A
CS: Update
SC: Observe A [token = 0]
CS: Content A [0] --> SR: Notification Resource A
....
RS: Unsubscribe A (observe list empty)
RS: Subscribe B (observation list id re-use)
CS: Content A [0] --> SR: Notification Resource B!!!!!!!!!!!!!!!!!! (client sent content for resource A)
....
CS: Update
SC: Observe B [0]
CS: Content B [0] --> SR: Notification Resource B (back to normal once update is received)